This invention relates to electron multipliers, and particularly to a multiplier structure which includes a high energy electron filter.
Display devices have been proposed in which electron multipliers operated in a feedback mode are used to provide current to light up a cathodoluminescent screen. For example, see U.S. Pat. No. 3,904,923, entitled "CATHODOLUMINESCENT DISPLAY PANEL", issued Sept. 9, 1975 to J. Schwartz. In one such structure, the electron multiplier includes at least two vanes having a plurality of parallel dynodes disposed in staggered relation thereon with a cathode at one end. This structure is further described in copending application, Ser. No. 672,122, filed Mar. 31, 1976. In this structure, electrical potentials of increasing magnitude are applied to the successive multiplying dynodes so as to produce an electron beam at the multiplier output. Generally, the electron multiplier has an open structure to allow feedback which results in sufficiently high loop gain to produce sustained electron emission.
In order to vary the screen brightness in such a structure, it is necessary to include a set of modulation electrodes at the multiplier output. The simplest modulation structure operates as a gate wherein the modulation electrodes are made sufficiently negative so as to turn back that part of the multiplier output which is not desired at the screen. However, in order to achieve a high degree of brightness modulation, e.g. 100:1, with reasonable voltages (voltages equal to, or less than, the voltages between successive multiplying dynodes), it is necessary to filter out of the multiplier output all electrons except those electrons which originate from the last dynode. The reason that some form of filtering is necessary is that electrons which originate from earlier dynodes and skip the subsequent multiplier dynodes before reaching the multiplier output are highly energetic. These highly energetic electrons require high modulation voltages to prevent them from reaching the screen.
Thus, it would be desirable to develop a high energy electron filter for the previously described multiplier structure. Although structures such as grids and wires can be used in a high energy electron filter, these structures increase the complexity of construction. In addition to this structural constraint, any workable filter must also meet the practical electron optical constraint: namely that the filter must be closed enough to prevent the passage of high energy electroncs but, the surrounding electrical fields must be such so as to efficiently steer low energy electrons therethrough. As a result, it would be particularly desirable to develop a simple, easily constructed structure for such a filter which would also be compatible with the techniques employed in constructing the multiplier itself.